The bloodline gets cut in half every time a new generation comes (unless you marry a sibling or cousin... *shudder*). It's been a long time since the Old Kingdom collapsed and since William Black defeated the King and Queen of Blades, so obviously, the bloodline would be quite thin. Also, in Fable II, you are able to master all Strength, Skill and Will. However, the three Heroes specializing in their attribute are still stronger than you in that principle of Heroes.
I'm thinking that Theresa is pretty normal in terms of Strength, and obviously terrible in Skill (hell, she's blind), but is a master in Will. That's the thing that is keeping Scythe, and probably herself alive. She has enough Will that she can see the future.
The Hero of Oakvale would be a master of all three (unless you decide to focus on only one or two), and would not have nearly enough Will to keep himself alive and immortal.
As a side note, while playing with Trainers, I maxed my Strength and Skill, had no spells and set my age to 18. I found out that although I was only 18, I looked like a rotting old man (65). However, when I leveled my Will Power once, I looked 18 again. And also, I don't remember what I had in my stats, but at age 65, I looked 18.
